The uniquely human phenomenon of crying when we are overcome by emotion developed as a means to communicate our feelings before the emergence of language, an influential scientist claims.

Michael Trimble, a professor of neurology, suggests that there must have been a time in our evolution when tears took on a meaning beyond their simple bio-mechanical function - keeping the eyeballs moist.

He has written a new book - Why Humans Like To Cry - that attempts to shed light on the mystery of why our species is the only one in the animal kingdom to shed tears of anguish.

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Why do we cry? Shedding tears in response to emotion seems to be a uniquely human phenomenon. A UCL professor's new book claims that it emerged as a way to communicate before oral language

The work, according to its publisher, offers a wide-ranging discussion of emotional crying, looking at its physiology as well as its evolutionary past.

Biologically, tears are needed to keep the eyeball moist and they contain proteins and other substances to keep it healthy and fight infections.

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In every other animal that seems to be extent of their function, but in humans, crying takes on a whole new, additional significance.

We can shed tears of joy and tears of anger and for a whole range of other emotions. But, most commonly, we shed tears of sadness.

Anguish: Professor Michael Trimble that the emergence of emotional crying is connected with the dawning of self-consciousness

Professor Trimble, of University College London's Institute of Neurology, said it was this uniquely communicative nature of human crying that led him to investigate the phenomenon.

'Humans cry for many reasons,' he told Scientific American. 'But crying for emotional reasons and crying in response to aesthetic experiences are unique to us.

'The former is most associated with loss and bereavement, and the art forms that are most associated with tears are music, literature and poetry.

'There are very few people who cry looking at paintings, sculptures or lovely buildings. But we also have tears of joy the associated feelings of which last a shorter time than crying in the other circumstances.'

Professor Trimble said he hopes his
work will release many, especially men, from the reluctance they feel to
be seen shedding tears.

They are a natural repsonse to not only suffering, but also to feeling compassion for others, he said.

He
added: 'We should not be afraid of our emotions, especially those
related to compassion, since our ability to feel empathy and with that
to cry tears, is the foundation of a morality and culture which is
exclusively human.'

Assessing the phenomenon from the perspective of neuroscience, Professor Trimble suggests that the crying in this way must have emerged in humans at a specific evolutionary turning point.

He believes that the emergence of emotional crying is connected with the dawning of self-consciousness and the development of a theory of mind - when early humans first realised their peers were also self-conscious beings.

This, he claims, led to the realisation that the self and others can suffer, feel sadness - and disappear.

'Attachment emotionally to others, with the development of sophisticated facial gestures associated with suffering, and with loss and bereavement ensued,' he said.

Why Humans Like To Cry is published by Oxford University Press, priced at £16.99 for hardback.